The present invention relates generally to devices for storage and retrieval of electronic data, and more specifically to disk drives for the storage and retrieval of information in computer systems, particularly portable computers and other computers for which a highly compact data storage medium is desirable.
The continuing trend toward portable computing systems is creating demands for storage devices that are extremely rugged, portable, economical and have a very thin profile. The predominant storage subsystem used in computers today consists of a removable volume device, called a floppy disk drive, and a fixed storage volume device, called a hard disk drive. The floppy disk drive functions as an economical software loading and distribution system, while the hard disk drive serves as the on-line, high speed, and high reliability data storage unit.
Common floppy disk drive mechanisms utilize a flexible Mylar disk that is coated with a thin layer of magnetic material. Recording/retrieval of data is performed by a ceramic head that houses a magnetic transducer. The Mylar disk is spun at a relatively low speed of approximately 300 rpm, and the ceramic head is penetrated into the plane of the disk, such that intimate contact is maintained between the head and the disk during the recording and retrieval of data. Such a technique is described by Tandon et al. in U.S. Pat. No. 4,151,573, the complete disclosure of which is incorporated herein by reference. This contact recording technology limits the performance of these devices.
Common hard disk drives utilize a rigid disk that is made of aluminum, glass or a ceramic material. The disk is much thicker that the Mylar floppy disk, and it is polished and coated with a thin layer of magnetic material. A small ceramic head, which is also polished and lapped, operates on this disk surface such that, as the hard disk spins at a speed of approximately 3600 rpm, a thin air film develops between the disk surface and the ceramic head. The thickness of this air film is governed by the speed of the disk, the geometry of the ceramic head, and an external force that is provided by the suspension which mounts this ceramic head onto the drive's actuator mechanism.
The major difference between floppy disk technology and hard disk technology is the fact that, the ceramic head in the floppy disk drive is in contact with the Mylar disk, while in the hard disk drive it is separated by a small and controlled air film, and it never contacts the magnetic coating during the recording process. Consequently, hard disk drives attain very high data recording and retrieval speeds, while data reliability is ensured by the non-contact operating conditions of the heads.
The technologies used by magnetic storage products generally fall into one of the following categories: Those that relate to the composition of the magnetic coating deposited on the substrate material, those that relate to the physics of the recording transducer, those that relate to the geometry of the ceramic head for contact or non-contact operation, those that relate to the design of the head/disk assembly that services the operating environment of the recording head, and those that relate to the recording/control electronics.
The current state of the art provides similar magnetic characteristics in the coatings that are available on the Mylar substrates of floppy disks and those that are deposited on the hard aluminum or ceramic disks. The processing methods for these substrates, however, are quite different. Mylar is available in sheet form, and it is coated using a wet process with a slurry consisting of magnetic particles, a binder material and a dry lubricant. After the coating process the disks are cut from the roll material and finished using appropriate burnishing operations. Newer processing techniques are becoming available, whereby, the magnetic material is deposited onto the Mylar film using a vapor deposition technique.
The hard disk process is much more elaborate, and requires the disk to be lapped and polished to a high degree of precision. This necessitates that the disks be larger in thickness than the Mylar disk, and have sufficient rigidity so that the polishing operations can be performed satisfactorily. The magnetic coating is then deposited onto the disk surface in a two stage operation, that first consists of depositing a hard film of nickel, followed by a thin layer of magnetic material that is sputter coated. The disks are finished by overcoating the magnetic film with a layer of carbon, to provide some lubricity and environmental protection for the corrosive magnetic coating. In the Mylar process, corrosion protection is achieved by coating the individual magnetic particles prior to mixing them in the slurry. Finally, due to the different processing methods the cost of a finished Mylar disk is significantly lower that the cost of a hard disk.
Recording transducer technology is quite similar between the two types of storage devices. However, product cost considerations have caused the lower performing, contact-recording floppy disk drive products to use transducer technologies that are typically below the state-of-the-art. Hard disk drives on the other hand, due to their greater market appeal, have successfully employed transducer technologies that are at the state-of-the-art. The most common recording transducer consists of a "soft" magnetic core material that is fabricated in a "C" shape. A highly controlled gap is developed between the two legs of the "C", such that, the magnetic flux fringes at this gap to influence the magnetic state of the particles in the disk coating. A coil of wire is wrapped around one leg of the "C" shaped transducer core, such that the magnitude of the current flowing in the coil governs the intensity of the fringing field in the gap for the recording process. The change of magnetic flux due to the passage of previously recorded information on the disk surface under the gap causes a current to flow in the head coil for the read back process.
Such transducer elements are manufactured by first building the "C" structure as a long bar of ferrite material and then machining each individual transducer from this long bar. Alternatively, a thin-film sputter deposition process is used wherein Permalloy layers form the legs of the transducer core, and copper layers form the coils in an intricate structure. This type of transducer structure is commonly called a thin film head. Thin film heads are more precise, requiring a number of processing steps, and consequently are more expensive. They are used in products that employ very large track densities, typically greater than 1800 tracks per inch, while the machined cores are cheaper and are limited by current machining precision to approximately 1800 tracks per inch.
The assembly methods and the design considerations for the contact-recording floppy disk drive vary significantly from those of hard disk drives. The hard disk drive mechanisms are designed to provide a reliable platform for the heads to maintain their non-contact operating condition with the disk surface. The disk is mounted onto a spindle motor, the axis of which is maintained parallel to the axis of the rotary actuator or perpendicular to the linear actuator, depending upon the specific design configuration. The wobble of the spindle motor, flatness of the disk, the clamping stresses of the disk on the spindle motor, and the runout of the spindle motor are controlled to maintain a flat spinning surface. The actuator mechanism is similarly designed with due consideration such that the ceramic head traverses along a radial trajectory in a plane that is parallel to the disk surface.
The head/disk assembly is manufactured in a very clean environment and contaminant particles are controlled so that they do not interfere with the operation of the non-contact ceramic head. For example, in the current state of the art, ceramic heads operate at a distance of approximately 0.1 micrometer (4 microinches) from the disk surface at the location of the recording transducer gap in the head structure. This means that in the current hard disk air bearing configurations, during operation, the entire trailing edge of the head, which is where the recording transducer is attached, is required to operate at this height above the disk surface. Submicron contaminant particles, if not properly controlled, can get wedged between the hard ceramic head surface and the hard disk substrate resulting in a head crash and loss of data. A filter material may be provided in this assembly for the air to be continually circulated through this filter for removal of contaminants and wear particles during the life of the disk drive product.
The result of all these operational requirements is that there is a limit to the economy with which a hard disk drive product can be manufactured. Furthermore, the entire mechanism must be designed with the necessary precision to cater to the operating boundaries of the non-contact head/disk interface. This precision also limits the ruggedness of the final product. Finally, due to the thickness of the hard disk, and the configuration of the ceramic head assemblies, there is a limit to the number of disks that can be packaged within a given product vertical height.
Floppy disk drives, on the other hand, operate in an open environment and are manufactured under less stringent conditions, resulting in a significantly lower product cost. The flexibility of the Mylar storage medium and the construction of the ceramic heads used in these disk drives give them a greater degree of ruggedness than hard disk products. However, these very operating conditions along with the contact recording methods limit the performance and storage capacity achieved in these devices.
In rotating memory products, information is recorded in concentric rings, called tracks, that span from the outer diameter of the disk to an inner diameter. The control electronics drive a head positioning actuator mechanism, which commonly utilizes a coil of wire placed in a magnetic field, so as to position the recording transducer in the ceramic head directly over a specific track. The recording heads are mounted on a structure that is connected to the coil of the actuator mechanism and supported by bearings in an appropriate fashion so as to have freedom of movement in a radial trajectory over the disk surface. The positioning servo control system derives its information from pre-defined bursts of information recorded along with stored data on all the tracks. This type of control system is called an "embedded servo control system" in the industry. Another servo control system is also employed in products that have more than two disk platters, where all the tracks on a specific disk surface are pre-recorded with a unique and dedicated servo pattern, and the head that operates on this surface is continuously reading this position information. The control electronics in these products sample the pre-recorded servo information from the respective track and corrective action is initiated as position errors are detected by the control loop.
Information is recorded in these drives in small segments that are 512 bytes long; these are called data sectors. The information recorded in each data sector is uniquely identified by a label called a header, and appended by additional bytes that contain a specific error correction algorithm. The heads, once positioned over the desired track, write or read information from some specific data sectors. The signal generated by the heads is amplified and processed by electronics and then assembled in a local buffer memory located in the drives electronics, prior to being passed on to the host computer system. The logic of the servo control system as well as the initiation of a read or a write operation, buffering of the data and then transfer of this data to the host computer is performed by a local microprocessor and electronic circuits mounted on the disk drive printed circuit board (PCB).
Hard disk drives operate at significantly higher speeds than floppy disk drives, and at the current state of the art, track densities of hard disk drives are 10 to 17 times larger than those used in floppy disk drives.
Product cost considerations dictate the electronic integrated circuit (IC) technologies employed by these respective disk drives. The trend towards miniaturization has resulted in greater integration of the electronics, as well as the development of IC chips that have a low vertical height and a small foot print. Furthermore, this trend has also resulted in more controller functions to be incorporated in the disk drive subsystem. These electronic design concepts allow a greater degree of flexibility in terms of data formats and device specific functions, while the external system is relieved of these tasks, and interface standards are maintained at a higher level.
Due to the extensive cleanliness requirements in hard disk drive products, all the electronics are incorporated on a PCB mounted under the head/disk assembly structure, resulting in an undesirably thick package for the entire device. In floppy disk drives, a mechanism is required to handle the insertion and ejection of the floppy diskette. This mechanism occupies considerable space in the head/disk area, thereby, the electronics are packaged under the drive casting, also creating an excessively thick package.
The interface types that are in common use today are the IDE and the SCSI standards. A new interface standard is becoming popular called the PCMCIA standard for credit card-type removable devices. At the present time the PCMCIA standard has two definitions for the vertical height of the removable peripheral device, namely, a version I accommodating devices that are 3.3 mm tall and a version II that covers devices that are 5.0 mm tall. Hard disk drive products currently cannot be designed to fit these form factors. Consequently, expensive semiconductor based units are servicing these needs in the market place.
Attempts have been made in the past to create products that use floppy disks that operate at high speeds, such that the heads can develop a non-contact condition with a single side of these disks. One commercial device is called a Bernoulli disk drive. This device depends upon the air pressure developed between the spinning flexible disk and a stationary backing plate to support the flexible disk, thereby giving it the necessary stiffness and stability to support hard disk-like operating characteristics. The recording head is penetrated into the plane of this disk from the side of the backing plate, as described by Losee and Norton in U.S. Pat. No. 4,414,592, the complete disclosure of which is incorporated herein by reference. This head has a very complex geometry and is quite costly. So far this technique has not been successfully extended to products with disk sizes that are smaller than the standard 51/4 inch product form factor.
Other attempts have been made to stretch a flexible disk over a plastic hub structure similar in construction to a drum. Standard hard disk heads were flown on the exterior flexible disk surfaces of this device. However, it has been found that the separation distance of the non-contact head is directly related to the value of the tension in the disk surface. Variations in tension would result in significantly different recording performance of the product. This dependence on disk tension in these products created problems in realizing this technology in any viable production volume.
A data storage and retrieval device is therefore desired which would have the high-speed, high storage characteristics and reliability of a hard disk, but with the low cost manufacturability and ruggedness of floppy disk drives. In addition it would be desirable if the device were extremely compact so as to meet the specifications of the PCMCIA standard. Preferably, the device should be adaptable to current computer systems without requiring specially-designed electronic modifications or additions to the existing computer hardware. Most desirably, the device should be removable to permit multiple storage units to be interchanged.